PinchZoom: allow the view to stay at fractional zoom levels

[aAXEe]

Currently the ol.interaction.PinchZoom zooms to the next whole-number zoom after the pinch completes.
This is of course good for classic image tiles but not for vector maps. A concrete user request for this: http://stackoverflow.com/questions/40882212/openlayers3-fractional-zoom-on-touch-device-using-pinch-gesture

I suggest to add a parameter like keepFractionalZoomLevel to the ol.interaction.PinchZoom constructor so that the user can choose whether to keep the fractional zoom or not.

I already tested this idea: aAXEe#1
A fiddle with a working example: https://jsfiddle.net/wm78prro/7/

[ahocevar]

Since wheel zooming now also does not restrict to integer zoom levels any more, I think such a change would make sense. Would you be able and willing to create a pull request?

[aAXEe]

As discussed in #6224 keeping the fractional zoom is now default.

To get the old behavior set the new constrainResolution parameter.